Several vehicle vision applications that utilize an image sensor to either control a vehicle function or provide an image to a driver have been proposed. Example applications include exterior light control, rain sensing, obstacle detection, collision avoidance, adaptive cruise control, lane departure warning, and night vision systems. Many image sensor technologies can provide for a wide interscene dynamic range by adjusting exposure between frames. High intrascene dynamic range, the range of brightness of objects which can be detected within a single frame, is more technically challenging to achieve. Examples of CMOS image sensors having high interscene and intrascene dynamic range are given in U.S. Pat. Nos. 6,175,383; 6,115,065; 6,008,486 and published U.S. patent application 20020057845. Additionally, high dynamic range image sensors are available from Pixim, Inc. of Mountain View, Calif. and Fill Factory, Inc. of Belgium. Charge coupled device (CCD) based high dynamic range image sensors are also available from numerous suppliers.
Most image sensing technologies providing high dynamic range involve construction of a relatively more complex pixel than traditional image sensors of lower dynamic range. As a result, such sensors are typically more expensive.
Vehicular night vision systems are currently being manufactured and proposed to improve night time driving safety by providing a supplemental view of the forward scene as imaged by a near infrared (NIR) or far infrared (FIR) image sensor assembly. Such systems have the advantage of being more sensitive than human vision and sensitive to radiation not visible to the human eye. An overview of currently available night vision technologies is contained in University of Michigan Transportation Research Institute publication UMTRI 2002-12, by Kåre Ruman, entitled Night Vision Enhancement Systems: What should they do and what more do we need to know, the content of this publication is incorporated in its entirety herein by reference.
The most significant problem with known vehicular vision systems is the washout and blooming that occurs from oncoming vehicle headlamps or other lamps within the scene. For conventional image sensors, this results from the fact that the image sensor pixels do not possess the dynamic range to sense both bright headlamps and the faint objects in the scene. If the sensitivity of the imager is set sufficiently high for adequate imaging of the scene when illuminated by a NIR light source, oncoming headlamps will substantially saturate the area of the imaged oncoming vehicle and severely limit the ability to see faint objects close in proximity to the oncoming vehicle. If the imager's sensitivity is set such that oncoming headlamps do not saturate the image, faint objects in the scene are not visible, thus negating most benefits of the night vision system. Since a goal of night vision systems is to provide nighttime forward vision substantially superior to the human visual system, even in the presence of oncoming traffic, this problem must be solved.
The high dynamic range image sensors described in the previously incorporated references solve the primary problem of limited pixel dynamic range. Another problem, electronic blooming, is also substantially solved with the use of known high dynamic range CMOS image sensors. Electronic blooming occurs in some image sensors when electrons generated in one pixel spill over into neighboring pixels, resulting in bright objects being imaged as a much larger object than they actually are and thus inhibiting the ability to resolve other objects in close proximity to the bright object. The pixel structure of these imagers inherently greatly reduces electronic blooming thus alleviating this problem. Anti-bloom drains are also added to pixels to further inhibit blooming. While the term blooming is commonly used to describe this phenomena, the more specific term electronic blooming will be used herein to distinguish from optical blooming or other types of scattering which have a similar effect on the resultant image. The more generic term blooming will be used herein to refer to the effect of a bright object being imaged larger than the actual object, regardless of whether the cause of the blooming is electronic, optical, or otherwise.
Due to the large variance in scene illumination and intensity of imaged light sources in vehicular applications, vision systems having a high dynamic range are desirable. What is needed is a vehicular vision system, comprising a high dynamic range, capable of detecting objects within a scene that have substantially varying brightness levels.